In recent years, portable electronic devices have been developed one after another, and as power sources for the portable electronic devices, secondary batteries have been playing a key role. In particular, recently, a secondary battery with a higher capacity and a lighter weight has been demanded, and as a secondary battery meeting such a demand, a lithium secondary battery using lithium as an electrode reactant has been put to practical use.
As a cathode active material of the lithium secondary battery, lithium cobalt oxide (LiCoO2) is widely used. It is because lithium cobalt oxide has a layer structure which is advantageous to diffuse lithium ions, and the structure is stable for insertion and extraction of lithium ions, so high characteristics can be obtained. However, in recent years, there has been a demand for the use of the lithium secondary battery in a wider temperature range or the use of the lithium secondary battery with a high current; therefore, demands for characteristics have become more difficult.
Therefore, a large number of attempts to improve characteristics by substituting another element for cobalt in the lithium cobalt oxide, or adding another element have been made. For example, Patent Literature 1 discloses a cathode active material capable of improving stability at high temperature by substituting a first element being at least one kind selected from the group consisting of aluminum (Al), chromium (Cr), vanadium (V), manganese (Mn) and iron (Fe) and a second element being at least one kind selected from the group consisting of magnesium (Mg) and calcium (Ca) for a part of cobalt. Moreover, Patent Literature 2 discloses a cathode active material capable of improving load characteristics, low-temperature characteristics and thermal stability by coprecipitating a zirconium compound at the time of synthesizing lithium cobalt oxide so that zirconium (Zr) is present on particle surfaces of lithium cobalt oxide.
[Patent Literature 1] Japanese Unexamined Patent Application Publication No. 2001-319652
[Patent Literature 2] Japanese Unexamined Patent Application Publication No. 2004-311408